Machines used in on-road and off-road locations may experience loss of traction. Such machines include motor graders, wheel harvesters, rotary mixers, wheel skidders, four-wheel drive vehicles, quarry construction trucks, large mining trucks, wheel loaders, wheel tractor scrapers, and articulated trucks. Certain machines, such as articulated trucks, have a front cab and a rear dump body hinged together by a joint for relative movement about a vertical axis. Each of the front cab and rear dump body includes at least one wheel set having at least one left wheel and at least one right wheel. Each axle may be rigid as articulated trucks generally steer by the angle between the front cab and rear dump body. The angle between the cab and the dump body may be determined by hydraulic rams in response to steering wheel input. Each wheel set may further include a differential which allows the respective wheels of a wheel set to rotate at different angular velocities, thereby allowing the machine to turn.
Differentials often include a differential clutch for limiting or overriding the differential to reduce wheel spin (i.e., traction control during acceleration) or wheel slip (i.e., anti-lock braking during deceleration). During acceleration, for example, if one of the wheels of a wheel set loses traction, an open differential will normally reduce torque delivered to the non-spinning wheel, thereby limiting the overall driving torque delivered to the wheels. The differential clutch, however, can override the differential to increase the amount of torque transmitted to the non-spinning wheel. For example, the differential clutch may include interposed plates and a piston configured to compress the plates together, thereby transferring torque from a machine drive shaft to the axles coupled to the wheels. The differential clutch may be used in a similar manner to minimize wheel slip during deceleration.
Some differential clutches permit partial clutch engagement to allow greater control over the amount of torque delivered to the wheels. In the foregoing example, the differential clutch piston may be pressure modulated so that the amount of torque transferred to the wheels may be a function of the amount of piston pressure applied to the plates. Thus, engagement of the differential clutch can be controlled to reduce wheel spin and slip, thereby improving torque control in low traction environments, albeit at the cost of a reduction in turning capability.
Existing traction control systems generally compare wheel speeds across an axle and apply a brake or clutch if the wheel speed difference is beyond a predetermined threshold. However, such control systems do not fully account for natural wheel speed differences that occur while steering or turning. In particular, some systems observe wheel spin or slip ratios and activate corrective controls when a predetermined threshold has been exceeded. Other systems observe wheel acceleration and activate corrective controls when a specific acceleration has been reached. There are also systems which observe both wheel spin or slip ratios and acceleration, and activate corrective controls when either predetermined threshold has been exceeded. These conventional systems may use state machine based control strategies. Such a system monitors wheel spin, slip or acceleration against predetermined thresholds in one state, and transitions to another state when the thresholds have been exceeded and so forth. By relying solely on wheel speed and transitioning between discrete states, the dynamics of wheel control change appreciably for different operating speeds.
Therefore, there is a need for a control system which optimizes traction and accounts for natural wheel speed differences that occur while steering or turning.